The discovery of the J/Psi in 1974 changed the world of particle physics forever! The discovery of the fourth quark was great evidence in support of the quark model of hadrons and Cabibbo’s quark mixing.

14 is for 14TeV, what we had planned for the LHC. So this video is all about how we glimpsed into the distant and not so distant future of particle physics. We need to keep looking ahead, the LHC is not the end of the journey!

Some links from the CERN public page about the accelerator complex and history:
Accelerator complex:
http://public.web.cern.ch/public/en/research/AccelComplex-en.html
http://public.web.cern.ch/public/en/research/SPS-en.html
http://public.web.cern.ch/public/en/research/LEP-en.html
http://public.web.cern.ch/public/en/LHC/LHC-en.html
http://public.web.cern.ch/public/en/research/CLIC-en.html

On Monday at 6 AM CERN time, the LHC ended its collisions of protons for 2012, and in fact until 2015, when the “long shutdown” for energy upgrades is completed. [There will be heavy-ion collisions in early 2013, but the details are beyond my expertise.] Here’s what appeared on the LHC status screen:

It’s the end of an era for the LHC, the end of what we might someday call “Run 1″, the period of first beams at (relatively) low energies, when we got our first glimpse of a new energy scale, and gathered just enough data to see the first glimmerings of (maybe) the Higgs boson. Considering how long we waited for the start of Run 1 — nearly twenty years from the first concepts for the the LHC and its detectors — it is rather amazing that we’re now at the end of the run, after a mere three years.

Still, it’s been a great three years. Here is the plot that captures the whole story:

This is the integrated luminosity recorded by CMS, essentially the number of collisions that the experiment observed, year by year. Remember all the excitement of the first data in 2010? That turned out to be a tiny amount of data compared to what we have recorded since; while we made very good use of it at the time, it was just hinting at the future success of the LHC. And even after the great advances in 2011, by the start of June 2012 we had recorded more data this year than we had in all of last year. Once again, all the experimenters thank the LHC team for the excellent performance of this still new machine.

The last few days of the proton run were spent looking towards the future. Since there won’t be any more proton collisions for two years, it’s important to do some tests that can guide our thinking about how to operate the LHC in 2015. So far, the LHC has run with “50 ns bunch spacing”; that is, the minimum time between bunch crossing is 50 nanoseconds. (Remember, the LHC beam is not continuous, but “bunched”, with a large number of protons close together in the beam, followed by a 50-foot gap before the next bunch in the beam.) This week, the LHC experimented with 25 ns bunch spacing, and even allowed the experiments to take a little bit of data in this mode on Saturday night and Sunday morning. Obviously, with the shorter bunch spacing, you can have beam collisions happening twice as often, and that means that you could potentially achieve the same total number of collisions with fewer protons per bunch. That’s good for the experiments, as each event that we record will have fewer collisions in it, making it easier for us to reconstruct what went on. With 25 ns spacing, we’d probably need less computing capacity and calibration and the like would be easier. But from the accelerator perspective, it is easier to operate the LHC with 50 ns spacing, and the machine operators can’t guarantee that they could provide as much integrated luminosity at 25 ns spacing as the could with 50 ns. Thus, it was important to take some time to understand how to operate the LHC this way. It’s ultimately up to the LHC managers to decide what the best mode for operations is. From the experiment side, it would be easier for us to have 25 ns spacing, but we wouldn’t want to do that at the cost of less data, and perhaps missing a chance of a discovery as a results.

Meanwhile, what does a 3000-member collaboration do with itself when there is no data to record? (Besides sending and reading email.) Quite a lot. First, there are a number of upgrades, repairs and improvements to be made on the detector in the next two years. There is a carefully choreographed dance to be performed in the collision hall, where the CMS detector must be opened up for access to the different components, and the schedule for all the work to be done could be pretty tight. There are also preparations to be made for how we analyze the data in 2015. The environment will be a lot like in 2010: we’ll be at a new beam energy, and in a physics environment that we’ve never seen before, so we’ll have to be ready for anything that might appear in the data. And we will continue our studies of the fabulous three years of data already recorded. During the past three years, the collaborations have released multiple papers on particular topics, with increases in the amounts of data analyzed each time and improvements in analysis techniques. But the next round of papers will use the full dataset, and there won’t be any “next” papers. The analysis techniques then must be the best possible; there won’t be another shot for improvements, as the next word will be the final word, at least until 2015. This too will take a lot of effort from the scientists.

Congratulations to everyone on a successful Run 1, and let’s look ahead to a busy shutdown and an exciting Run 2 beyond!

As the Large Hadron Collider (LHC) is preparing to shut down for the end of the year holidays, the LHC experiments presented on Thursday morning a summary of the last three years of operation. For CMS and ATLAS, the highlight was of course the discovery of what looks more and more like the Higgs boson.

The certainty for the presence of a new boson has been reinforced. As Sara Bolognesi, speaking on behalf of the CMS collaboration, put it: “The signal is so strong, the probability of having it wrong is as low as the chance of flipping a coin 40 times and getting 40 heads in a row”. Marumi Kado, representing ATLAS, showed that even when using a single decay channel, the signal is strong enough to claim a discovery. Hence, the focus is now on finding the exact properties of this new boson to reveal its identity.

ATLAS showed their first results on the spin and parity of the new boson. The parity seems positive, as expected for the Standard Model Higgs boson, reaching the same observation as CMS. But the jury is still out on the value of its spin although the results are more compatible with 0, the value expected by the Standard Model, but a value of 2 is still possible. A clearer answer might come once the 23 inverse femtobarns of data delivered this year by the LHC will have been processed and combined for the two experiments.

What’s new on the more-and-more-Higgs-like new boson? CMS showed the first results on a Higgs boson decaying into a Z boson and a photon. This decay channel should be very small unless there are contributions from processes predicted by theories going beyond the Standard Model, and these could be huge. Nothing is seen so far but this is a promising avenue.

A few facts are nevertheless puzzling. For example, ATLAS measures two different masses when the Higgs decays to two photons as opposed to four leptons, the two decay channels giving the best precision on the mass measurement.

Each one of these decay channels represents one way the Higgs boson can break apart. It is very much like making change for one dollar. No matter if you give the change with coins of ten, twenty or fifty cents, the total sum should always add up to one dollar. As it stands, it is as if ATLAS obtains $1.05 and $0.95 when adding up all the coins, despite having checked each channel with extreme scrutiny for a possible mistake.

This is most likely due to a statistical fluctuation since the data gives only one mass value in the global combination but it might take more data than is at hand to resolve this apparent discrepancy. CMS obtains similar masses in both channels but the results need to be updated with more data for the two-photon channel.

Another slightly intriguing fact: both experiments measure more Higgs boson decays into two photons than what is predicted by the Standard Model. I summarized the situation in the table below.

The error margins are still fairly large which means more data will be needed to sort it all out. The LHC will undergo a major upgrade starting in March 2013, to restart at higher luminosity and higher energy beginning of 2015. It takes a lot of patience to do high energy physics!

Pauline Gagnon

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